Ice Sheets Were Terrifyingly Thick 20,000 Years Ago

This neat visualisation by Randall Monroe of XKCD fame shows just how thick the ice sheets covering our planet were 20,000 years ago. Towering 3,300 meteres above the Montreal skyline, these things were huge. So huge it's difficult to picture them, though this cartoon goes a long way in helping. Thank goodness they melted. [XKCD]